BIOL 150A - Class 6: Mendel's Laws & Deviations

Vocabulary practice flashcards covering Mendelian laws, extensions to Mendelian genetics (incomplete dominance, codominance, pleiotropy, polygenic inheritance), and linked genes based on BIOL 150A Class 6 notes.

Principle of Allelic Segregation

A Mendelian principle stating that alleles of a gene pair separate during gamete formation, such that half ($\frac{1}{2}$) of the gametes carry one allele and half ($\frac{1}{2}$) carry the other allele.

Classical Gene Definition

A unit of inheritance operationalized in classical genetics by yielding a $3:1$ phenotypic ratio in the F2 generation of a monohybrid cross.

Monohybrid F2 Phenotypic Ratio

The classic $3:1$ ratio of dominant to recessive phenotypes resulting from a cross between two heterozygous F1 monohybrid individuals.

Monohybrid F2 Genotypic Ratio

The $1:2:1$ genotypic ratio ($1 \text{ PP} : 2 \text{ Pp} : 1 \text{ pp}$) produced in the F2 generation from a monohybrid cross of two heterozygotes.

Principle of Independent Assortment

A Mendelian principle stating that alleles of different genes separate independently of one another during gamete formation in meiosis.

Dihybrid Cross Phenotypic Ratio

The classic $9:3:3:1$ ratio of four phenotypic classes observed in the F2 generation when two dihybrid individuals ($RrYy \times RrYy$) are crossed under independent assortment.

Hypothesis of Dependent Assortment

A refuted hypothesis proposing that alleles for two different traits are inherited together as a linked unit, which would predict a $3:1$ F2 ratio rather than the observed $9:3:3:1$ ratio.

Linked Genes

Genes located close together on the same chromosome that tend to move together during meiosis, violating the principle of independent assortment.

Incomplete Dominance

An inheritance pattern in which a heterozygote expresses an intermediate phenotype between the two homozygous parents because neither allele is completely dominant.

Snapdragon Flower Color Inheritance

A classic example of incomplete dominance where crossing true-breeding red ($RR$) and white ($rr$) snapdragons (Antirrhinum majus) yields all pink ($Rr$) F1 plants, and an F2 phenotypic ratio of $1 \text{ red} : 2 \text{ pink} : 1 \text{ white}$.

Codominance

An inheritance pattern in which two different alleles for a gene are simultaneously and fully expressed in the phenotype of a heterozygous individual.

Multiple Alleles

The existence of three or more allelic forms of a gene within a population, although any individual diploid organism carries a maximum of two alleles.

ABO Blood Group System

A human blood classification system governed by a single gene with three alleles ($I^A$, $I^B$, and $i$) that exhibits both codominance ($I^A$ and $I^B$) and complete dominance over $i$.

Type AB Blood Genotype

The heterozygous genotype $I^A I^B$, which results in the co-expression of both Antigen A and Antigen B on the surface of red blood cells.

Type O Blood Genotype

The homozygous recessive genotype $ii$, which produces neither A nor B antigens on the surface of red blood cells.

Universal Donor Blood Type

Blood Type O, which lacks both A and B antigens on red blood cells, preventing reaction when transfused into recipients with anti-A or anti-B antibodies.

Universal Recipient Blood Type

Blood Type AB, which contains neither anti-A nor anti-B antibodies in plasma, allowing individuals to receive any ABO blood type.

Pleiotropy

A genetic phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits across an organism.

Sickle-Cell Allele Genotype

The homozygous genotype $Hb^S Hb^S$, which produces abnormal sickle-cell hemoglobin that crystallizes and deforms red blood cells into a sickle shape.

Sickle-Cell Pleiotropic Effects

The multiple systemic effects caused by the single homozygous $Hb^S$ gene, including organ failure (kidney, heart, spleen, brain damage) and secondary symptoms (pain, fever, joint problems, physical weakness, anemia, pneumonia).

Polygenic Inheritance

An inheritance pattern in which a single continuously-varying phenotypic trait is controlled by the additive effects of multiple independent genes.

Continuous Variation

A phenotypic distribution (such as adult human height) that exhibits a continuous gradient of values across a population, shaped by polygenic inheritance and environmental influences.

Crossing Over (Crossover)

The exchange of chromosomal segments between non-sister chromatids of homologous chromosomes during Meiosis I that breaks linkage between genes.

Recombinant Chromosomes

Chromosomes carrying new combinations of alleles resulting from crossing over between homologous chromosomes during Meiosis I.

Non-Recombinant Chromosomes

Chromosomes that maintain the original parental combination of alleles because no crossing over occurred between the tracked gene loci.

Recombination Frequency

The percentage of recombinant gametes produced during meiosis, which reflects the relative physical distance between two linked genes on a chromosome.

Gene Mapping

A classical genetics procedure that uses recombination frequencies between linked genes to determine their relative positions and distances along a chromosome.

Map Unit / Recombination Distance

A unit of measure on a genetic map where $1$ map unit corresponds to a 1\text{ }\format{cctaylor}{\%} recombination frequency between two linked loci.

Fruit Fly Genetic Map

A chromosome map of Drosophila melanogaster depicting gene positions based on recombination values, such as short aristae ($0$), black body ($48.5$), cinnabar eyes ($57.5$), vestigial wings ($65.5$), and brown eyes ($104.5$).

Fruit Fly Black Body Locus

A specific mutant locus in fruit flies located at map position $48.5$ on the genetic map, causing a black body phenotype instead of gray.

Phenotype

The observable physical, physiological, or biochemical traits of an organism resulting from the interaction of its genotype and environment.

Genotype

The genetic composition or specific set of alleles present in an organism's DNA for a given gene or set of genes.

P Generation

The parental generation consisting of true-breeding homozygous organisms used to initiate a genetic cross.

F1 Generation

The first filial generation of offspring produced directly by crossing two distinct individuals from the parental (P) generation.

F2 Generation

The second filial generation of offspring produced by self-fertilization or interbreeding among F1 hybrid individuals.

Heterozygous

An organism or cell having two different alleles for a specific gene locus (e.g., $Pp$ or $RrYy$).

Homozygous

An organism or cell having two identical alleles for a specific gene locus (e.g., $PP$, $pp$, $RRYY$, or $rryy$).

Punnett Square

A grid-based visual diagram used to calculate and predict the expected genotypes and phenotypes of offspring from a genetic cross.

Meiosis I Homologous Chromosome Alignment

The cellular mechanism in Metaphase I and Anaphase I of meiosis where random orientation of homologous pairs leads to the independent assortment of unlinked genes.

Plant Disease Resistance Inheritance

A trait in plants where fungal pathogen resistance is typically inherited via simple Mendelian genetics (e.g., susceptible $RR$ crossed with resistant $rr$).